This invention is directed to a process for preparing polyesters which process comprises reacting a dihydric phenol with a diester derivative of an aromatic diacid in the presence of from about 10 to about 60 weight percent, based on the weight of the polyester produced, of a processing aid having a boiling point of greater than about 200.degree. C. under polyester forming conditions.
Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl) propane, also identified as Bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Many processes have been described in the literature for the preparation of polyesters. One such process is the diphenyl ester process.
The diphenyl ester process, generally involves two steps. In the first step, diphenyl iso- and/or terephthalates are prepared by the reaciton of phenol with iso-or terephthalic acids, or mixtures thereof, at a temperature of about 180.degree. to about 350.degree. C. in the presence of a catalyst. In the second step, the diphenyl iso- and/or terephthalates are reacted with bisphenol-A in the presence, optionally, of a catalyst at a temperature of from about 200.degree. to about 350.degree.. However, heretofore, this process has been carried out by several different methods all suffering from serious deficiences as illustrated in the following patents:
U.S. Pat. No. 3,395,119 describes the preparation of copolyesters by reacting (a) bisphenol A with (b) 70-10 mole percent of a diaryl terephthalate, (c) 25-89 mole percent of a diaryl isophthalate and (d) 1-5 mole percent of a diaryl carbonate present in the reaction mixture. The volatile reaction products are removed until a copolyester having a relative viscosity of greater than 1.6 is obtained. However, this process is applicable to the production of polyesters having ester and carbonate linkages and not to polyarylates having carbonate-free linkages. Also, as described in Examples 1 and 3 of U.S. Pat. No. 3,395,119, the polyester tends to develop color. Further, a vacuum is required during polymerization.
U.S. Pat. No. 3,972,852 describes the preparation of polyesters by reacting in a molten state (a) an aromatic dicarboxylic acid or an ester-forming derivative thereof, with (b) a diphenol to prepare a polymer, which is then further polymerized in the solid phase to prepare the linear aromatic polyester. The reaction is carried out in the presence of at least one dihydroxy compound or a carboxylic acid ester thereof. The Examples demonstrate carrying out the reaction in the presence of a catalyst. The catalysts include titanium compounds, an antimony compound or an organo tin compound. However, in order to obtain good results the process requires the use of an aliphatic diol as a correactant. Aliphatic diols have deleterious effects on polymer thermal stability. Also, in this patent, the preparation of the polyester is achieved by a solid phase polymerization which is a very cumbersome process.
U.S. Pat. No. 4,124,566 describes a process for preparing polyesters by a first step of esterifying (a) a difunctional carboxylic acid containing at least 60 mole percent of an aromatic dicarboxylic acid, (b) 0 to 80 mole percent of an aliphatic diol and/or a dihydroxybenzene, and (c) an aromatic hydroxy compound until the degree of esterificatin reaches at least 80 percent. The second step comprises adding 0 to 80 mole percent of component (b) and (d) a bisphenol to the reaction product obtained in the first step and reacting the mixture. At least the first of the above steps is conducted in the presence of an aromatic hydrocarbon medium which has a boiling point of 105.degree. to 175.degree. C. The aromatic hydrocarbons are described as lower alkyl substituted benzenes. However, the use of a vacuum is required during polymerization.
Thus, the diphenyl ester processes for producing polyesters by the procedures of the aforediscussed U.S. Patent are generally unsuitable since they employ severe reaction conditions and produce highly colored and thermally unstable polymer.
Therefore, a need exists for a practical diphenyl ester process for producing high molecular weight polyesters with good color and thermal stability.
The utilization of from about 10 to about 60 percent of a processing aid is beneficial in that the viscosity of the system is decreased. This decrease in viscosity provides a faster reaction time since better mixing of the reactants occurs which allows the reaction to proceed under kinetic control. Additionally, by using the processing aid, reaction times are relatively short so that the polyester produced possess vastly improved color, as compared to those prepared under reaction conditions described in the prior art. Furthermore, the present process can be carried out at atmospheric pressure and therefore avoids the use of the costly equipment which is needed by the prior art processes which carry out the diphenyl ester process under vacuum.
Finally the use of the processing aid prevents the occurrence of undesirable "hot spots" which produce colored degradation products hich contribute to the instability of the produced polyester.
One problem which exists in the diphenyl ester process for producing polyesters is that when a dihydric phenol is reacted with a diester derivative of an aromatic diacid in the molten state codistillation of the diester and diphenol with the phenol occurs. This disrupts the stoichiometry of the reaction and the polyester produced is not of acceptable molecular weight. The use of a processing aid alleviates this problem; it also prevents sublimation and allows for the selective separation of phenol from the other coreactants.